In animals, an inflammatory response occurs when tissues are injured by bacteria, trauma, toxins, heat, or other agents, which can be collectively referred to as “Inflammatory Agents.” The nature and character of a given inflammatory response is regulated by the complex interaction of a variety of pro-inflammatory or anti-inflammatory stimulators or mediators, which are synthesized and released by tissue. Known species of pro-inflammatory or anti-inflammatory stimulators or mediators include, but are by no means limited to, cytokines, nitric oxide, thromboxanes, leukotrienes, platelet-activating factor, prostaglandins, kinins, complement factors, superantigens, monokines, chemokines, interferons, free radicals, proteases, arachidonic acid metabolites, prostacyclins, beta endorphins, myocardial depressant factors, anandamide, 2-arachidonoylglycerol, tetrahydrobiopterin, and chemicals including histamine, bradykinin, and serotonin. The discovery of new (i.e., previously unrecognized) species of pro-inflammatory or anti-inflammatory stimulators or mediators occurs almost daily.
The nature and intensity of inflammatory responses differ, depending on the site which has been invaded, and on the character of the Inflammatory Agent(s), and the interaction of pro-inflammatory or anti-inflammatory stimulators or mediators involved.
The inflammatory response, when regulated and localized, is beneficial. However, if not regulated and generalized, the inflammatory response can cause significant tissue injury and even death.
For example, cytokines are a class of proteins produced by macrophages, monocytes, and lymphocytes in response to viral or bacterial infection, as well as in response to T cell stimulation during an immune response. Cytokines are normally present in very low concentrations in the blood or tissues.
The structures and activities of cytokines have been the subject of many studies. It has become apparent that cytokines possess a wide spectrum of immunological and non-immunological activities. It is also apparent that cytokines affect diverse physiologic functions, such as cell growth, differentiation, homeostasis and pathological physiology. It is clear that cytokines have multiple biological activities and interact with more than one cell type. Cytokines are also known to be capable of stimulating their own synthesis, as well as the production of other cytokines from a variety of cell types. This phenomenon is called the “cytokine cascade.”
Cytokine cascades are associated with systemic changes arising from infection and tissue injury and, in this context, they serve a myriad of biological functions. For example, various cytokines, categorized as the interleukins (IL), interferons (IF), and tumor necrosis factor (TNF), are produced during immune and inflammatory responses. These cytokines beneficially control various aspects of these responses. In this situation, the cytokine cascade mediates normal host defense responses, cell regulation, and cell differentiation.
However, it has been observed that the function of cytokine production can become disordered. This can lead to the presence of larger than normal concentrations of cytokines. When the cytokine cascade becomes disordered, there can be a rapid extension and amplification of the intended localized host response in such a way that only one or a few initiating stimuli trigger the eventual release and participation of scores of host mediators. Although a number of features of the host response assist in fighting off invasion, an overly robust or poorly modulated endogenous response can rapidly accelerate to produce other profound alterations in host homeostasis at the cellular, tissue, and systemic levels. As a result, cytokine expression in a region of the body where tissues or organs are legitimately subject to bacterial infection or an immune response challenge, can, when disordered, lead to unwanted destruction of healthy tissue elsewhere in the body. Larger than normal concentrations of certain cytokines can cause disease and other deleterious health effects, some of which can be lethal.
For example, a disordered cytokine cascade that leads to the increased presence of the cytokines IL-1 and TNF can, alone or in combination, cause a state in animals clinically identical to “septic” shock. It is recognized that septic shock arises due to the individual, combined, and concerted effects of a large number of cytokines. It is a condition inflicting more than 450,000 Americans every year. Cytokine-induced septic shock can be brought about by infection by a variety of microorganisms, including not only bacteria but also viruses, fungi, and parasites. Septic shock can also be initiated by host response to invasion in general, such as by cancer or as a result of major surgery or trauma. Septic shock is a potentially lethal cytokine-mediated clinical complication against which there is no generally effective therapeutic approach.
One of the best studied examples of cytokine-induced septic shock is the case of infection by gram-negative bacteria. It is believed that the appearance of bacterial endotoxins, such as lipopolysaccharide (LPS), in the host bloodstream leads to the endogenous production of a variety of host factors that directly and indirectly mediate the toxicity of LPS. These host-derived mediators include many now well-recognized inflammatory cytokines, as well as endocrine hormones, in addition to a number of other endogenous factors such as leukotrienes and platelet activating factor. Among the interacting factors that together comprise the cytokine cascade, the cytokine TNF alpha is believed to be the most important identified to date. During the ensuing cytokine cascade, the mediators that appear early in the invaded host are thought to trigger the release of later appearing factors. Many of the cytokine mediators not only exert direct functions at the targeted tissues, but also at other local and remote tissues, where subsequent responses to other mediators produced during the cascade occur, and so on. The result, if unchecked, can be a multifaceted pathological condition, which is characterized most prominently by deleterious hemodynamic changes and coagulopathy leading to multiple organ failure and, often, to death.
Multiple attempts have been made and still many others are currently underway to block specific mediators of this response. These attempts have been relatively unsuccessful. Therapy aimed at single mediators cannot effectively attenuate the entire response. Furthermore, it is the duration rather than the intensity of inflammation that correlates best with outcome, in that the longer the duration of over-expression of proinflammatory cytokines the higher the mortality. Systemic inflammation results in organ injury which results in the prolongation of the inflammatory response and thus, more organ injury.
Less lethal but just as profound physiologic effects can occur as a result of abnormal production of certain cytokines, without the presence of exogenous bacterial toxins. As one example, cytokine TNF-alpha has been found to be an anti-tumor cytokine. As a result, TNF-alpha has been expected to be useful as an antitumor agent. However, it has been discovered that TNF-alpha is identical with cachectin, which is a cachexia-inducing factor. The disordered production of TNF-alpha has also been correlated with, not only septic shock, but the incidence of rheumatoid arthritis, adult respiratory distress syndrome (ARDS), the severity of viral hepatitis, myocardial ischemia, and the inhibition of myocardial contraction. Also, TNF has recently been shown to be involved in initiating the expression of human immunodeficiency virus in human cells that carry latent virus, which could be a contributing factor in the expression of latent AIDS virus in certain individuals. Furthermore, a correlation between the TNF level in the blood and blood pressure has also been observed. As TNF levels increase, blood pressure decreases, which can lead to serious complications such as kidney failure.
It has also been observed that TNF-alpha also has an activity of stimulating production of other types of cytokines, such as IL-1, etc. It is known that the cytokine IL-1 is an important agent for inducing and transmitting the systemic biological response against infection and inflammation. IL-1 induces the usual, desirable responses observed in inflammation in general, such as fever, increase of leukocytes, activation of lymphocytes, induction of biosynthesis of acute phase protein in liver. It also known that this cytokine has a strong antitumor activity.
However, when IL-1 is produced in abnormally larger amounts, it may contribute to the severity of chronic inflammatory diseases, such as rheumatoid arthritis. Thus, the abnormal activation of various cytokines such as the interleukins (IL) and tumor necrosis factor (TNF) is believed responsible for the tissue damage and pain that occurs in various inflammatory conditions like rheumatoid arthritis. In rheumatoid arthritis, levels of TNF, IL-1, IL-6 and IL-8 increase dramatically and can be detected in the synovial fluid. The cytokine cascade induced by expression of these cytokines results in depressed lipoprotein metabolism as well as bone and cartilage destruction.
As another example, the cytokine IL-6 plays an important role in antibody production in B cells. The cytokine IL-6 also is an important factor in body systems, e.g., the hematopoietic system, nervous system, and the liver, as well as in immune system. For example, IL-6 is effective for inducing proliferation and differentiation of T cells, inducing the production of protein at acute phase by acting on hepatic cells, and promoting the growth of cells in bone marrow.
However, it has also been observed that there is a correlation between the abnormal secretion of IL-6 and various disease states, e.g., autoimmune diseases, such as hypergammaglobulinemia, chronic articular rheumatism, and systemic lupus erythematosus; the abnormal state of polyclonal B cells, as well as in the development of the abnormal state of monoclonal B cells such as myeloma cells; Castleman's disease accompanied with tumor of the lymph nodes, for which the cause is unknown; primary glomerular nephritis; and the growth of mesangial cells.
As yet another example, in bacterial infections, cytokines such as IL-8 act as a signal that attracts white blood cells such as neutrophils to the region of cytokine expression. In general, the release of enzymes and superoxide anions by neutrophils is essential for destroying the infecting bacteria. However, if cytokine expression causes neutrophils to invade, for example, the lungs, release of neutrophil enzymes and superoxide anion can result in the development of adult respiratory distress syndrome (ARDS), which can be lethal.
Despite their diverse and myriad functions, all cytokines share one common feature. They are all within a narrow size and molecular weight range of 8 to 28 kilodaltons. This size characteristic is extremely important for the clearance of cytokines from the blood. In this range, cytokines are effectively cleared by the liver and also the kidney, which clears all proteins below 50 kilodaltons in size. An imbalance between cytokine production and cytokine removal can cause damage to the liver and kidney.
In disease states where the kidney has failed—which is often the case in septic shock—hemodialysis or hemofiltration membranes are used as substitutes for the glomerular membrane of the kidney. However, artificial membranes are severely limited in their ability to clear cytokines from the blood due to their inadequate porosity. In fact, the predominant mechanism by which these membranes remove cytokines in clinical practice is not filtration, but rather nonspecific surface adsorption (J. Am. Soc Nephrol 1999 April; 10(4): 846-53, Cytokine removal during continuous hemofiltration in septic patients, De Vriese A S, Colardyn F A, Philippe J J, Vanholder R C, De Sutter J H, Lameire N H). Typically these membranes have 0.5 to 2 square meters of surface area available for adsorption that becomes saturated within the first 30 to 90 minutes of treatment (Biomaterials 1999 September; 20(17):1621-34, Adsorption of low molecular weight proteins to hemodialysis membranes: experimental results and simulations, Valette P, Thomas M, Dejardin P).
It is therefore clear that pro-inflammatory or anti-inflammatory stimulators or mediators, such as cytokines but by no means limited to cytokines, have the potential for both desirable physiologic results and undesirable physiologic results, depending upon the robustness and modulation of a particular inflammatory response. There is a need for straightforward and biocompatible devices, systems, and methods that serve to reduce or otherwise modulate levels of pro-inflammatory or anti-inflammatory stimulators or mediators in instances where abnormal levels of or unregulated or excessive interaction among such materials exist or can be expected to arise.